In Newton's classical mechanics, time played the role of a monotonically increasing evolution parameter. Einstein rejected the Newtonian concept and instead identified time as the fourth coordinate of a space-time four-vector. Today, scientists are considering different concepts of time as a means of resolving incompatibilities between relativity and quantum mechanics. Some view time as an emergent property of a system rather than a fundamental property, while others consider two temporal variables. The purpose of this book is to examine the role of time in modern physics so that the reader gains an increased awareness of time and its place in our understanding of nature.